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.\" Copyright (c) 1998-2004 Proofpoint, Inc. and its suppliers.
.\" All rights reserved.
.\" Copyright (c) 1993 Eric P. Allman. All rights reserved.
.\" Copyright (c) 1993
.\" The Regents of the University of California. All rights reserved.
.\"
.\" By using this file, you agree to the terms and conditions set
.\" forth in the LICENSE file which can be found at the top level of
.\" the sendmail distribution.
.\"
.\"
.\" $Id: smrsh.8,v 8.23 2013-11-22 20:52:00 ca Exp $
.\"
.TH SMRSH 8 "$Date: 2013-11-22 20:52:00 $"
.SH NAME
smrsh \- restricted shell for sendmail
.SH SYNOPSIS
.B smrsh
.B \-c
command
.SH DESCRIPTION
The
.I smrsh
program is intended as a replacement for
.I sh
for use in the ``prog'' mailer in
.IR sendmail (8)
configuration files.
It sharply limits the commands that can be run using the
``|program'' syntax of
.I sendmail
in order to improve the over all security of your system.
Briefly, even if a ``bad guy'' can get sendmail to run a program
without going through an alias or forward file,
.I smrsh
limits the set of programs that he or she can execute.
.PP
Briefly,
.I smrsh
limits programs to be in a single directory,
by default
/etc/smrsh,
allowing the system administrator to choose the set of acceptable commands,
and to the shell builtin commands ``exec'', ``exit'', and ``echo''.
It also rejects any commands with the characters
`\`', `<', `>', `;', `$', `(', `)', `\er' (carriage return),
or `\en' (newline)
on the command line to prevent ``end run'' attacks.
It allows ``||'' and ``&&'' to enable commands like:
``"|exec /usr/local/bin/filter || exit 75"''
.PP
Initial pathnames on programs are stripped,
so forwarding to ``/usr/ucb/vacation'',
``/usr/bin/vacation'',
``/home/server/mydir/bin/vacation'',
and
``vacation''
all actually forward to
``/etc/smrsh/vacation''.
.PP
System administrators should be conservative about populating
the /etc/smrsh directory.
For example, a reasonable additions is
.IR vacation (1),
and the like.
No matter how brow-beaten you may be,
never include any shell or shell-like program
(such as
.IR perl (1))
in the
/etc/smrsh
directory.
Note that this does not restrict the use of shell or perl scripts
in the sm.bin directory (using the ``#!'' syntax);
it simply disallows execution of arbitrary programs.
Also, including mail filtering programs such as
.IR procmail (1)
is a very bad idea.
.IR procmail (1)
allows users to run arbitrary programs in their
.IR procmailrc (5).
.SH FILES
/etc/smrsh \- directory for restricted programs
.SH SEE ALSO
sendmail(8)
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